LONDON, Ont. — Ontario's dry summer followed by recent rains has created a deadly hazard for the province's farmers — a gas that can kill within seconds.

The province's ministry of agriculture has issued an alert to farmers to watch for silo gas, which sometimes forms as a reddish brown haze but can also be invisible. It has a bleach-like odour.

Inhaled, the gas — nitrogen dioxide — can cause permanent damage to the lungs and sudden death.

There have already been a couple of reports of farmers being injured, but no fatalities, said Peter Johnson, a crop specialist with the ministry.

"It is real this year," Johnson said.

The gas is typically produced in corn plants when there has been a long dry period, followed by rain. That happened last week.

When the corn is loaded into tower silos, the gas, which is heavier than air, can flow down the chutes into feed rooms.

The greatest risk is in the first 12 to 60 hours after a silo has been filled.

According to the ministry, inhaled silo gas mixes with body moisture to form nitric acid, which causes severe burning of the lungs and the rest of the respiratory system.

Victims often collapse and other people attempting to rescue them can be overcome.

"Farmers exposed to silo gas should get immediate medical attention," the ministry warning says.

Silo gas killed a 14-year-old farm boy near Waterloo, Ont., in 2008 when he was overcome by fumes at the top of a 20-metre silo.

The ministry says farmers should take the following precautions:

— Do not enter a silo during the risk period without wearing an appropriate self-contained breathing apparatus

— Before entering the silo, ventilate it by running the forage blower for 30 minutes and leave it running while inside

— Ventilate the silo room and chute

— Post appropriate warning signs, and keep people and animals away